Protection of Metals from Corrosion

Because corrosion is fundamentally an electrochemical process requiring oxygen, moisture and a favourable electrochemical driving force, every practical protection method works by removing one of those ingredients. Five standard strategies are used:

1. Coating with paint. A layer of paint on the metal surface physically blocks oxygen and moisture from reaching the metal, cutting off the corrosion reaction at its source.
2. Galvanising. The metal (typically iron) is coated with a layer of zinc. Because zinc is a stronger reducing agent than iron — it is more easily oxidised — zinc corrodes preferentially instead of the iron underneath, even if the zinc coating is scratched; the iron is protected as long as any zinc remains nearby to sacrifice itself first.
3. Cathodic protection. Unlike galvanising, this technique does not require the entire metal surface to be covered with a protective metal. Instead, a more easily-corroded metal such as magnesium or zinc is deliberately connected to the metal being protected and allowed to act as a sacrificial anode, while the metal to be protected (e.g. iron) is forced to act as the cathode. Since oxidation (corrosion) only happens at the anode, the iron is protected while the sacrificial magnesium or zinc corrodes instead.
4. Passivation. The metal is treated with a strong oxidising agent, such as concentrated HNO₃, which forms a thin, tightly-adherent protective oxide layer directly on the metal's surface. This oxide layer then physically shields the metal underneath from further oxidation, much like the natural Al₂O₃ layer that protects aluminium. …